Solved!

Originally Posted by phoenix

You've already been given the answer (last year), use the settings mentioned above by Lynner.

Hi Phoenix,

The reply he gave connects your Zimbra server to an existing LDAP server (e.g Active Directory, which I already did).
But I decided to apply some of the settings to Outlook, I used the Zimbra IP as the server name, unchecked this " this server requires me to logon" I clicked on more settings, and checked custom base and inserted: " ou=people,dc=mydomain,dc=com".
This time, the query worked.
Thanks a lot Lynner and Phoenix.